Any thoughts on what might be the issue is appreciated.

2019 3.5L Ecoboost with 106,000 miles. I’ve had the truck around 6 months and have replaced the oil/filter, air filter, and verified all fluid levels are normal. Still need to replace plugs and coils as I’m not sure when or if they were ever replaced. The truck has the dreaded “cold start rattle” related to VCT phasers.

I’ve noticed while cruising under light load, when only minimal boost is required, that the turbos seem to spool up and down erratically about once every 2-5 seconds, as if they can’t decide on a where to stabilize at. RPM does not fluctuate, but the turbo whistling sound and associated acceleration does. When more than light power is required, the turbos seem to perform normally, and power is good. These symptoms seem to occur only when the truck is warmed up well. It’s especially noticeable and annoying when pulling a 5000lb trailer with cruise control on, as boost is often just barely required at freeway speeds and continually cycles erratically.

I haven’t found a thread yet that matches this issue perfectly…I’m thinking wastegate solenoid maybe? BOV diaphragm leak? Turbo pressure/MAF sensors dirty? I’ll be changing the plugs and coils shortly, but I have a feeling that is not going to fix it. Any insight is appreciated!

 

Sounds like compressor surge. Check your piping, vac lines, BOV, etc.

 

Unrelated to your topic, but skip the expense of replacing coils.
They aren't bad, especially all 6 of them aren't bad. You are probably right in that they haven't been replaced, because people don't replace coils until one of them fails, then they replace one.

 

Maybe OP meant coils and boots? But otherwise I agree with you.